For most plants and animals, we know that increasing connectivity is a key strategy for mitigating the negative effects of habitat fragmentation. In contrast, we know far less about how microorganisms—bacteria, archaea, fungi, and protists —may experience connectivity. This is despite microorganisms’ important roles in mediating soil fertility and plant and animal health, as well as increasing concerns that humans may be altering microbial communities.
Mony, C., Uroy, L., Khalfallah, F., Haddad, N. and Vandenkoornhuyse, P. 2022. Landscape connectivity for the invisibles. Ecography 8: e06041.
In a recent review, researchers argue that developing a connectivity framework for microorganisms requires more research that takes microbes’ distinct natural histories into consideration.
Many microorganisms are host-associated, such as ectomycorrhizal fungi on plant roots or gut-specializing bacteria. In these cases, the patches of habitat are the hosts themselves, and connectivity for these microbes depends on the births, deaths, and distributions of their hosts across landscapes.
However, microbes are not mere passengers; instead, they may actively influence their hosts’ health and sometimes their movement or growth. For example, some fungi increase the elongation or branching of their host plant, enhancing the likelihood of contacting another plant individual. Therefore, understanding microbes’ dispersal among hosts must consider the physiology of the host, the microbe, and their interactions with each other.
The small size of microbes shapes how and why they disperse. Microbes experience their environment, such as a change in pH or nutrient concentration, on very small scales. Thus, dispersal from one suitable habitat patch to another can range from distances of millimeters to kilometers. For example, recent research showed that a unicellular protist, which moves by waving hair-like projections called cilia, was more likely to disperse down a 5 cm long corridor than a 10 cm long one.
Other microbes move passively along currents of air or water. Since corridors influence wind speed and direction, corridors designed for animals and plants may affect the dispersal of passively-dispersed microbes. The size of corridors relevant for microbes should depend on the microbe’s dispersal mechanism, as well as the range of environmental conditions that it can tolerate.
The lifespan of microorganisms also influences how they experience connectivity. Many microbes are alive only for minutes to days. For microbes living on a shorter timescale, dispersal from one patch to another may take generations, during which these microbes may acquire adaptations that help them survive. Such microbes may respond to changes in connectivity quickly.
Other microbes exist in a reversible state of low activity, where they are protected from poor environmental conditions, called dormancy, for hours to thousands of years. Dormant microbes may respond to habitat fragmentation or increased connectivity only after many years.
Overall, microbial traits such as dispersal mode, host-association, environmental tolerance, and dormancy shape their movement across a landscape. Therefore, considering functional connectivity, which takes into account how the natural history of a species influences dispersal, is the most relevant approach for microorganisms. While more research is needed on microbial traits and connectivity, the importance of microbes to nearly every aspect of ecosystem health clearly means that such research should be a priority in the context of increasingly rapid habitat fragmentation.
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